A person familiar with the wood pulp industry would know that there are different methods for making pulp from wood. Some pulp-making methods involve chemical processes or treatments that free pulp fibers from wood chips. Among these are the well-known Kraft and sulfite processes. Other methods utilize mechanical means for freeing the pulp fibers. These are commonly called groundwood processes. Still others involve a combination of mechanical and chemical processes, where the pulp fibers are primarily freed by mechanical means aided by chemical treatment. These are called "chemi-mechanical" processes.
The different pulp-making methods briefly described above each have their own advantages and disadvantages which need not be described here. However, their differences present variations in the efficiency of utilizing wood raw materials, and in the quality of pulp that is produced, the latter governing the suitability of the pulp for different uses. These same differences also present different environmental problems as to the disposal of their waste products.
The present invention is related to the disposal of waste products from pulp mills that fall within a general class known as TMP mills, which utilize "thermo-mechanical pulp-making" processes. The invention is more particularly related to CTMP or BCTMP mills, which are subclasses of the TMP class. As the skilled person would know, the CTMP and BCTMP acronyms stand for "chemical thermo-mechanical pulp-making" and "bleached chemical thermo-mechanical pulp-making", respectively.
Referring in particular to BCTMP mills, pulp is produced there in what is essentially a three-stage process. The first stage involves a pre-bleaching step; the second is a refining step; and the third is a final, bleaching step.
In the pre-bleaching step, wood chips are treated with a solution containing sodium hydroxide and hydrogen peroxide. The wood chips are then drained prior to refining. In the refining step, pulp fibers are freed by a mechanical grinding action that is done in the presence of a solution containing sodium hydroxide, sodium sulfite, and other chemicals including sodium silicate and certain wetting agents. After refining, the resultant pulp is then drained and washed. This is followed by the final, bleaching step, where the pulp is bleached in a solution of sodium hydroxide and hydrogen peroxide, followed by raining and washing the pulp one last time.
A pre-bleach liquor is produced after the wood chips are drained in the pre-bleach step; pulping liquor and wash are produced after the pulp is drained and washed in the refining step; and bleach liquor and wash water are produced after the pulp is drained and washed in the final bleaching step. These various liquors and wash make up the Waste effluent that is discharged by the BCTMP mill. A typical BCTMP mill may produce approximately 2,200 gallons of such effluent every minute. Of this amount, organic and inorganic contaminants make up approximately 1% to 3% by weight. The rest is water.
The contaminants make the effluent unsafe for dumping into the environment without some form of prior treatment. Traditionally, BCTMP effluent has been biologically treated in large holding ponds or sloughs, where bacteria metabolize and decompose the organic contaminants over a period of time before further discharge into nearby lakes or rivers. Although the inorganic contaminants remain largely untreated by this method (approximately one-half of the contaminants are inorganic), it has been considered an environmentally sound form of treatment in most cases.
One drawback to biological treatment is that it requires that large quantities of water be available or located near the mill for discharge of the effluent. For this reason, it has been impractical to situate BCTMP mills in regions where there are no large lakes or rivers. A second drawback is that biological treatment produces large quantities of sludge, which creates a secondary disposal problem in and of itself.
As will become apparent, an object of the invention disclosed here is to free BCTMP mills from their past dependency on biological waste treatment, and/or to eliminate otherwise any mill dependency or requirement for large quantities of water for effluent discharge. Another object of the invention is to provide an improved or better form of waste water treatment.
The invention accomplishes this, in part, by utilizing evaporation technology to separate the water in the effluent from the contaminants, and by reusing or recycling the water back to the bleaching and washing portion of the BCTMP process. Although such technique sounds simple enough, its implementation creates other kinds of problems.
A first problem is that, when processing the rate of effluent flow described above via evaporation (approximately 2,200 gallons a minute), separating pure water from the contaminants results in a by-product of many tons per hour of concentrated waste or concentrate. Such waste also must be disposed of one way or another.
The present invention handles such waste by feeding it into a recovery boiler where it is incinerated. Not only does this provide the advantage of a clean form of solid waste disposal, but it also provides a source of steam that can be used elsewhere in the mill.
Incineration in a recovery boiler also enables recovery of valuable inorganic chemicals that are reusable, in the BCTMP mill, or in other mills, and is especially well-suited for reuse in Kraft mills. These advantages notwithstanding, a recovery boiler has its own unique requirements that create another problem directly affecting evaporation of the effluent.
In order to efficiently burn the concentrate, it is desirable that the effluent first be concentrated to a very high level so that it can independently support combustion. As a practical matter, the waste should be concentrated to approximately 65% to 70% solid matter by weight. Unfortunately, this concentration requirement affects the quality or purity of the water evaporated from the effluent.
Evaporation and subsequent condensation is typically accomplished in stages, where the effluent is passed from one evaporator to another, with the level of waste in the concentrate increasing through the stages. A person familiar with BCTMP mills would know that the early evaporate from each evaporator is impure, because it contains methanol. This is removed by stripping, which is a standard process that is well-known. However, the purity of the condensate produced, even after methanol removal, will decline further as the effluent's concentration increases above a certain level.
In particular, the condensate becomes contaminated by excessively high carryover of certain volatile organics present in the effluent, the most notable of which is acetic acid. If, for example, the level of acetic acid in the condensate becomes too high, then the condensate becomes chemically unacceptable for reuse in the BCTMP process. It also may not be acceptable, or safe, otherwise for dumping directly into the environment, if that is desired.
As will become apparent, the present invention provides a unique resolution to the above situation, where a solution to a given problem creates another problem of a different kind. That is to say, the invention provides a way to evaporate BCTMP effluent until it reaches a waste concentration level that will support combustion in a recovery boiler, while, at the same time, it controls the quality of the water evaporated from the effluent so that it may be reused. How the invention accomplishes this is described below.